A conventional driving apparatus for a three-phase alternating current (AC) synchronous motor has, as disclosed in JP 3223842 (U.S. Pat. No. 6,137,704 and U.S. Pat. No. 6,320,775) for example, an inverter circuit for outputting three-phase AC currents of the motor and an inverter control circuit for controlling the inverter circuit.
In the inverter circuit, three sets of paired transistors, which are connected in series in each phase, are connected in parallel one another between a positive bus line and a negative bus line. A diode is connected to each transistor in parallel and in reverse bias direction in the inverter circuit.
A direct current (DC) power source is connected between the neutral point of the stator coil and the negative bus line of the inverter circuit. A capacitor is connected between the positive bus line and the negative bus line of the inverter circuit.
The inverter control circuit is configured to drive the six transistors to perform switching operation, so that the three-phase AC currents are supplied to the motor based on a voltage difference between the positive bus line and the negative bus line.
When the transistor is turned on, a current flows from the DC power source to the stator coil so that the stator coil stores energy based on the current.
When the transistor is turned off, the stored energy is discharged as a current, which flows from the stator coil to the positive electrode of the capacitor through the diode provided and the positive bus line. Thus, by the switching operation of the six transistors, the three-phase currents are supplied to the stator coil and the capacitor is charged.
When the capacitor is charged with the current, which flows from the stator coil, thermal loss is generated in the stator coil.
As the charge to be stored in the capacitor by the switching operation of the transistors increases, the thermal loss of the stator coil increases. As a result, efficiency in driving the motor is degraded.